1. Field of the Invention
The present invention relates to a current detection circuit for a power semiconductor device, and in particular to a current detection circuit for detecting a current running in a power semiconductor device such as a diode or a transistor, for example, a bipolar transistor, a MOSFET (metal oxide semiconductor field effect transistor), or an IGBT (insulated gate bipolar transistor).
2. Description of the Related Art
An IGBT module installing an IGBT and a free wheeling diode (hereinafter referred to simply as an FWD), which are types of power semiconductor devices, is used in a power conversion apparatus such as an inverter or a DC (direct current) chopper circuit.
In order to control a circuit of such a power conversion apparatus, generally an output current must be detected. For the output current detection, the following two methods are usually employed.
(1) A method using a current detection device for example, a current transformer or a DCCT (direct-current current-transformer).
(2) A method using a resistor called a shunt resistor for current detection.
FIG. 4 shows an example of construction of a conventional three phase inverter apparatus using DCCTs. Referring to FIG. 4, the DCCTs 105 utilize a magnetic core with a ring shape and a Hall element as shown in FIG. 18 in Patent Document 3, in which the electric current is detected by detecting a magnetic field generated by the current flowing in the main circuit wiring passing through the magnetic core. The inverter 101 shown in FIG. 4 is provided with the DCCTs 105 that are disposed around the main output wiring and a control circuit 102 that receives the detected current value.
FIG. 5 shows a relationship among an output current, an IGBT current and an FWD current in the lower arm of one phase of the inverter apparatus shown in FIG. 4. Provided that the direction of the output current is positive when the current flows out of the inverter apparatus, a positive current flows in the FWD 202 of the lower arm of the inverter apparatus and then a negative current flows in the IGBT 201 of the lower arm of the inverter apparatus as shown in the right side of FIG. 4. Here, the negative current is an electric current drawn into the inverter apparatus. The output current is repetition of these positive and negative currents.
Of the IGBTs and FWDs composing an inverter apparatus, there are types of IGBTs and FWDs called ‘sense IGBTs’ and ‘sense FWDs’ that perform a current detection function. The following describes a function of an example of a semiconductor device, specifically an IGBT element, with a sense function. FIGS. 6(a) and 6(b) show circuit symbols (FIG. 6(a)) and equivalent circuits (FIG. 6(b)) of the IGBT element with a sense function. The circuit symbols are represented as those in FIG. 6(a) and the equivalent circuits are represented by those in FIG. 6(b).
An ordinary IGBT consists of several thousand to several tens of thousands of cells having the same structure. A portion of the cells are utilized for current detection. The portion of the IGBT cells for current detection is called as ‘a sense region’ and the other IGBT cells are called as ‘a main region’. In general, a ratio Nm/Ns of the number of cells Nm in a main region to the number of cells Ns in a sense region is set to several thousands, wherein Nm and Ns are positive integers. Although collector terminals of the main region and collector terminals of the sense region are made in common, emitter terminals are separated to a main emitter terminal (referred to simply as a main terminal) and a sense emitter terminal for current detection (referred to simply as a sense terminal).
Likewise, in the case of an FWD, a portion of the FWD chips is separated for current detection; anode terminals are separated to a main anode terminal (simply referred to as a main terminal) and an anode terminal for current detection (simply referred to as a sense terminal). An equivalent circuit of an FWD including a sense FWD is similar to the one as shown in FIG. 6(b) for an IGBT including a sense IGBT. The symbols in FIG. 6(b) for a sense IGBT are changed for the sense FWD to: an internal resistance of the main region Rdm0, an internal resistance of the sense region Rds0, a threshold voltage of the main region Rdthm0, and a threshold voltage of the sense region Vdths0.
FIG. 7 shows simulation waveforms of an output current and currents in the IGBT and the FWD in one phase of the inverter apparatus of FIG. 4. The waveform of electric current includes: an output current, an electric current flowing in the IGBT of the lower arm, and an electric current flowing in the FWD of the lower arm. As shown in FIG. 7, the current in the IGBT of the lower arm flows only when the output current is negative, that is, in the direction of current being drawn into the inverter apparatus. On the other hand, the current in the FWD of the lower arm flows only when the output current is positive, that is, in the direction of current being flowing out from the inverter apparatus. Negative output current is never generated at the time when positive output current is generated; and positive output current is never generated at the time when negative output current is generated.
FIG. 8 shows a conventional example of the construction of an inverter apparatus that uses a current detection circuit employing shunt resistors 106 commonly for both IGBTs 201 and FWDs 202. The current detection circuit of FIG. 8 has the shunt resistors 106 connected in the lower arm of the inverter 101. The output current flows in the lower arm when the IGBT 201 in the lower arm is in the ON state, and a voltage drop in the shunt resistor 106 is detected by the control circuit 102 to detect the output current. A current detection circuit employing a shunt resistor is disclosed in Patent Document 1 (identified below), for example.
FIG. 9 shows a conventional example of the construction of an inverter apparatus that uses a current detection circuit employing shunt resistors 106 for IGBTs 203 with a sense function and further shunt resistors 106 for FWDs 204 with a sense function. This current detection circuit is provided with a shunt resistor 106 for each sense IGBT 203 and another shunt resistor 106 for each sense FWD 204. The current flowing in the shunt resistors 106 is detected and delivered to the control circuit 102. This type of current detection circuit is disclosed in Patent Documents 2 and 3 (identified below), for example. The shunt resistors 106 are individually connected to the sense IGBTs 203 and the FWDs 204 in the lower arm of the inverter 101 as shown in FIG. 9. The output current flows through the IGBTs 203 of the lower arm to the ground when the IGBTs 203 in the lower arm are in the ON state. The voltage drop in the shunt resistors 106 is delivered to the control circuit 102 to detect the output current. The output current flows through the FWDs 204 of the lower arm to the motor 104 when the FWDs 204 are in the ON state. The voltage drops across the shunt resistors 106 are delivered to the control circuit 102 to detect the output current.
In the ON state of the IGBTs 203 of the lower arm, an electric current flows through the sense terminal of the IGBT 203 corresponding, in principle, to the ratio of number of cells Nm/Ns as shown in Formula 1 below.Im/Is=(Nm+Ns)/Ns≈Nm/Ns  (Formula 1)where                Im is a main current, that is, the current flowing in the main IGBT,        Is is a sense current, that is, the current flowing in the sense IGBT,        Nm is the number of cells in the main region, and        Ns is the number of cells in the sense region.        
The sense current Is is detected by the shunt resistor Rs connected to the sense terminal of the IGBT 203 as shown in FIG. 9, and the main current Im is calculated based on Formula 2 below.
                                                        Im              =                            ⁢                                                (                                      Nm                    /                    Ns                                    )                                ·                Is                                                                                        =                            ⁢                                                (                                      Nm                    /                    Ns                                    )                                ·                                  (                                      Vs                    /                    Rs                                    )                                                                                        (                  Formula          ⁢                                          ⁢          2                )            
The method of detecting a main current Im using a shunt resistor Rs carrying the main current has a problem of a large loss in the shunt resistor. However, the above described method generates an insignificant loss in the shunt resistor, thus, eliminating the problems of decreased efficiency and enlarged resistor size.
Nevertheless, this method still has a problem of precision in the current detection. Inverter control generally requires a current detection precision of 1 to 2%. The method using the shunt resistor connected to the sense region exhibits such a low precision that cannot fill the above-mentioned requirement for current detection precision.
The low precision may be caused by the following two factors.
(a) A Factor of Difference in Characteristics of the Main Region and the Sense Region.
Equality between the ratio of magnitudes of current and the ratio of numbers of cells is only valid under the assumption that the cell characteristics are identical between the main region and the sense region. Actually, the characteristics are various and the ratio of the main current to the sense current is not constant. Thus, linear relationship cannot be supposed between the main current and the sense current.
(b) A Factor of a Shunt Resistance Effect
A voltage drop develops due to an electric current in the shunt resistor Rs connected to the sense terminal. As a result, an electric potential difference arises between the main terminal and the sense terminal, disturbing a constant ratio of the main current to the sense current.
FIG. 10 is a block diagram showing a construction of one phase portion of the inverter apparatus including the current detection circuit employing shunt resistors shown in FIG. 9. In the construction of FIG. 10, current detection circuits 210 employing a shunt resistor are connected to the sense terminal of the sense IGBT 203 and to the sense terminal of the sense FWD 204 in the lower arm.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2003-274667 (FIG. 7, in particular)    [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2009-268336 (FIG. 1, in particular)    [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2000-134955 (FIG. 1 and FIG. 18, in particular)
The conventional current detection methods described above have the following problems. The current detection method employing DCCTs as shown in FIG. 4 has problems that the method is generally expensive due to use of a Hall sensor element and a magnetic core, and that the output characteristics changes due to temperature variation around the DCCTs. Additional problem is that the size of the detector is large due to use of the magnetic core, which poses limitation on size reduction of a power conversion apparatus.
The current detection method employing a shunt resistor as shown in FIG. 8 has a problem that a power loss is generated in the shunt resistor, which decreases a power conversion efficiency of a power conversion apparatus. Additional problem is that the size of the resistor itself becomes large in order to allow a large loss, which poses limitation on size reduction of a power conversion apparatus.
The current detection method employing shunt resistors as shown in FIG. 9 has another problem that electric current measurement with high precision is impossible due to difference between the characteristic of the sense IGBT and the characteristic of the main IGBT. Also between the characteristic of the sense FWD and that of the main FWD.